1. Field of the Invention
The invention is directed to a fixation monitoring device for an ophthalmological instrument, having a fixating light source for visible light and optics for imaging the fixating light source on a fundus of an eye and to a method for monitoring fixation of the eye.
2. Description of Related Art
With some ophthalmological instruments the patient to be examined or treated needs to gaze in a defined direction. As a rule, this orientation is decisive for the correctness of measurement or therapy and, therefore, for successful treatment. This is particularly true for refractometers, wavefront aberrometers and refractive lasers.
Usually, a visible fixating target which is to be visually fixated by the patient is displayed in the visual field of the patient. Without further action, the operator of the device (i.e., the person performing the examination or treatment) would have to trust the patient to comply with this direction. However, particularly over longer periods of time, there is an increasing likelihood that the patient will fail to maintain the state of fixation. In particular, the human eye can carry out saccadic movements at a speed of up to about 600° per second so that drastic deviations of eye gaze direction can occur within a short time. Lack of certainty with regard to fixation often leads to dispersions in diagnostic or therapeutic findings. It is particularly problematic to maintain and monitor fixation in children, blind persons and persons with other kinds of disabilities. Accordingly, the operator needs the most objective possible information as to whether the patient actually fixates the fixating target or when this may no longer be the case.
To this end, different approaches are known from the prior art. For example, US 2006/0142742 A1 describes a device for ophthalmological treatment having a UV laser in which an additional light beam is provided for visual fixation which generates a light spot on the patient's retina. A camera records an image of a plane of the retina in the region of the fovea centralis (abbreviated as fovea hereinafter). In order to monitor fixation, this image is used for checking whether or not the light spot lies on the fovea. Further, an image of the pupil can be recorded in order to monitor fixation based on the centroid of the pupil or on the relative position of the light spot in relation to the pupil. This type of fixation monitoring has a number of disadvantages. First of all, a two-dimensional image sensor with a high pixel count is obligatory in order to achieve a sufficient accuracy in monitoring. Consequently, the image-taking frequency has an upper limit, for one, because of the lengthy readout of the sensor and, for another, because the subsequent evaluation of at least portions of the image is relatively long owing to the amount of data. The result is a relatively slow reaction to a loss of fixation with the consequent risks during a fast eye movement.
U.S. Pat. No. 6,027,216 describes a method for monitoring fixation in which the fundus is illuminated and radiation that is backscattered from the fundus is picked up by a polarization-sensitive detector. Based on changes in polarization between the illumination light and the backscattered light, it is determined whether the backscattering takes place at the fovea or on the rest of the retina. Backscattering on the fovea is used as an indicator of visual fixation on the part of the patient. It is disadvantageous that polarization-sensitive detection is complicated. In addition, the changes in polarization are minor compared to unwanted influences such as depolarization in the cornea and eye lens so that distinguishing between backscattering on the fovea and on the rest of the retina is fraught with relatively great uncertainty.